A modified phenolic resin that is an alternate copolymer of at least one phenolic compound selected from phenol, naphthols, and their derivatives and a compound having a divalent connecting group, said modified phenolic resin having a side chain attached to an aromatic ring having a hydroxy group, said side chain being represented by defined formula (1-1). The modified phenolic resin can be used as a hardener for epoxy resins and a cured product thereof has excellent adhesion and flame retardancy without impairing properties of conventional phenolic resins such as gel time, glass transition temperature, moisture absorption, and mechanical properties. The epoxy resin composition can provide excellent adhesion and flame retardancy as hardeners for semiconductor sealing epoxy resins, insulating materials for electrical/electronic components, and laminates (printed circuit boards). A prepreg containing a glass substrate impregnated with the epoxy resin composition, a laminate, and an electronic circuit board are also provided.